Most electronic products in use today generate heat. If unchecked the heat may result in reduced reliability of the product over time. Accordingly, measuring and controlling the thermal characteristics of a product are important design considerations. To resolve thermal issues, several proactive measures have been developed including reducing the speed at which a constituent processor of the product is running, engaging cooling fans, lowering a cellular network RF transmit power limit, dimming a display, etc. Many computer systems designed today require these proactive measures to provide adequate performance in order to not frustrate the user's experience with the device, under various thermal conditions. Temperature sensors may be embedded or integrated inside the housing of the product, to sense temperature at various locations. Closed loop thermal control or management algorithms may be developed in the factory based on data collected from these temperature sensors while running the device in its various normal operating modes. The algorithms are then stored as thermal control software (part of a thermal control subsystem) in each of the individual production specimens of the device. A processor runs this software during the in-the-field use, monitoring or collecting data from the temperature sensors over time and then analyzing the data and comparing to predetermined constraints, to make decisions on which thermal actions to take in order to effectively manage the thermal behavior of the device so as to maintain or regulate the temperature of a given location within a specified constraint.
Temperature sensors are often embedded within large integrated circuits, which can only be placed at a limited number of locations inside the computer system housing. Also, using a large number of discrete thermal sensors (e.g., thermocouples, thermistors) may in some cases be cost-prohibitive. Temperature sensors may be placed in various places within a computer system such as, for example, on the microprocessor (CPU) die, in the proximity of the CPU die (CPU Prox), on a motherboard, on a hard drive device, on a heat pipe, on a battery, on a track pad, etc. However, very often, a desired spot whose temperature needs to be carefully monitored or regulated in order to not exceed a specified limit may be located at a particular point where it is difficult to put a sensor, such as the bottom face of the case of a laptop computer system or the rear face (back plate) of a smart phone device. These points may be generally referred to as critical points or hotspots. Thermal models may be generated to accurately estimate the “virtual temperature” at these critical points, based on data received from temperature sensors located elsewhere in the device. See, for example, U.S. Patent Application Publication No. 2010/0094582 of Keith Cox, et al., “Method for Estimating Temperature at a Critical Point”, assigned to the same assignee as the present application (“Our Earlier Application”). Proactive measures may then be taken by the closed loop thermal control subsystem using this estimated temperature data (together with data from actual temperature sensors and data indicating current power consumption levels by various components in the device), to mitigate the thermal behavior at a critical point, such as turning on a system fan, lowering power supply voltage of a nearby component, and so on.
Thermal control is particularly challenging for a consumer electronic portable electronic device that is held and carried by its user while in operation, such as a cellular telephone, smart phone, digital media player and the like. Such devices are usually small and densely packed so that heat may not be easily dissipated. Furthermore, such devices are frequently in intimate contact with a user's skin so that customer satisfaction concerns arise if the thermal characteristics of the device are not properly managed. As a result, manufacturers of electronic products design the thermal control subsystem to limit the exterior surface temperature to levels that promote user comfort.